Alvaro Uribe has taken office for a second four-year term as president of Colombia. He was re-elected with 62 percent of the vote in May, is a close ally of the United States, and has successfully cracked down on violence and crime in Colombia. But he faces continued challenges in fighting drug traffickers and a guerrilla insurgency that has raged for more than four decades.

Security was tight for Monday's ceremony, with thousands of troops patrolling the capital.

During Mr. Uribe's first inauguration in 2002, rebel mortar attacks killed 21 people in Bogota. Last week, rebels staged a series of deadly attacks against security personnel.

Mr. Uribe, whose father was killed by rebels, has focused his efforts on fighting the country's largest rebel group, the Revolutionary Armed Forces of Colombia, or FARC.

He says he is not afraid to negotiate peace. "What worries me more," he said, "is falling short of that goal and instead seeing our gains in security eroded."

Bruce Bagley, chairman of the department of international studies at the University of Miami, says that, although 30,000 paramilitary fighters were demobilized during Mr. Uribe's first term, the president has had limited success against the leftist rebels.

"His goal with regards to the guerrillas is to defeat them sufficiently to force them to negotiate at the table," he said. "Whether that is possible or not, very much remains to be seen, but his first four years did not indicate that he had made a great deal of headway towards that."

He also says there is concern that ex-paramilitary fighters have resumed drug or other criminal activities.

Colombia is the worlds largest producer of cocaine, despite billions of dollars from the United States under the anti-narcotics Plan Colombia.

Michael Shifter, vice president for policy at the Inter-American Dialogue in Washington, says Mr. Uribe must also address the country's dire poverty.

"There's clearly major levels of poverty, particularly in rural areas that have been neglected, and they are also recruiting grounds for a lot of the illegal armed groups who join them not for ideological reasons, but for economic reasons," he said.

The United Nations says there are an estimated 2.5 million internally displaced people in Colombia. The U.N. refugee agency warns that some small ethnic groups could face extinction as their members flee their ancestral homes to escape the violence.